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You can connect to Exchange using POP and IMAP, so therefore you can use any mail client. I use Apple Mail on my Mac personally and still use Outlook on Windows, but I also use Thunderbird on occasion.

If you want something completely free and have access to Exchange from the outside you can also use Outlook Web Access.

POP and IMAP have to be enabled by your system administrator, and I've worked at more than 1 company where they are both disabled. Outlook uses something different to connect, and sometimes that is the only thing allowed. :(
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Jon TackaburyJul 17 '09 at 14:02

@Jon Agreed. However it can be done and with good motivation some administrators will allow it.
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DiagoJul 17 '09 at 14:17

You can enable IMAP and SMTP on exchange - convincing the administration is a social task.
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gimelJul 17 '09 at 14:08

@Jason S: Evolution can do appointment/calendar handling with exchange, even over a standard IMAP connection, if that's what you're after. There's a separate Exchange connection method which takes an OWA URL; I haven't managed to get that working yet though. The Microsoft-specific protocol you're talking about is MAPI, as far as I know. Evolution has a separate, newer MAPI plugin for exchange, too. That crashed badly for me, but it's still experimental, so give it some time.
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Lee BOct 27 '09 at 21:12